Downregulation of muscarinic- and 5-HT1B-mediated modulation of [H]acetylcholine release in hippocampal slices of rats with fimbria-fornix lesions and intrahippocampal grafts of septal origin

Adult Long-Evans female rats sustained electrolytic fimbria-fornix lesions and, two weeks later, received intrahippocampal suspension grafts of fetal septal tissue. Sham-operated and lesion-only rats served as controls. Between 6.5 and 8 months after grafting, both the [³H]choline accumulation and the electrically evoked [³H]acetylcholine ([³H]ACh) release were assessed in hippocampal slices. The release of [³H]ACh was measured in presence of atropine (muscarinic antagonist, 1 μM), physostigmine (acetylcholinesterase inhibitor, 0.1 μM), oxotremorine (muscarinic agonist, 0.01 μM–10 μM), mecamylamine (nicotinic antagonist, 10 μM), methiothepin (mixed 5-HT₁/5-HT₂ antagonist, 10 μM), 8-OH-DPAT (5-HT_1A agonist, 1 μM), 2-methyl-serotonin (5-HT₃ agonist, 1 μM) and CP 93129 (5-HT_1B agonist, 0.1 μM–100 μM), or without any drug application as a control. In lesion-only rats, the specific accumulation of [³H]choline was reduced to 46% of normal and the release of [³H]ACh to 32% (nCi) and 43% (% of tissue tritium content). In the grafted rats, these parameters were significantly increased to 63%, 98% and 116% of control, respectively. Physostigmine reduced the evoked [³H]ACh release and was significantly more effective in grafted (−70%) than in sham-operated (−56%) or lesion-only (−54%) rats. When physostigmine was superfused throughout, mecamylamine had no effect. Conversely, atropine induced a significant increase of [³H]ACh release in all groups, but this increase was significantly larger in sham-operated rats (+209%) than in the other groups (lesioned: +80%; grafted: +117%). Oxotremorine dose-dependently decreased the ([³H]ACh) release, but in lesion-only rats, this effect was significantly lower than in sham-operated rats. Whatever group was considered, 8-OH-DPAT, methiothepin and 2-methyl-serotonin failed to induce any significant effect on [³H]ACh release. In contrast, CP 93129 dose-dependently decreased [³H]ACh release. This effect was significantly weaker in grafted rats than in the rats of the two other groups. Our data confirm that cholinergic terminals in the intact hippocampus possess inhibitory muscarinic autoreceptors and serotonin heteroreceptors of the 5-HT_1B subtype. They also show that both types of receptors are still operative in the cholinergic terminals which survived the lesions and in the grafted cholinergic neurons. However, the muscarinic receptors in both lesioned and grafted rats, as well as the 5-HT_1B receptors in grafted rats show a sensitivity which seems to be downregulated in comparison to that found in sham-operated rats. In the grafted rats, both types of downregulations might contribute to (or reflect) an increased cholinergic function that results from a reduction of the inhibitory tonus which ACh and serotonin exert at the level of the cholinergic terminal.